This invention relates generally to hollow containers with inert or impermeable, i.e. glass-like, inner surfaces and more particularly to such containers produced by a plasma assisted application of a thin internal coating comprised principally of one or several inert inorganic substances, or layers of substances, such as silica or corrosion-resistant metal oxides or metals, and which can include carbon or an organic component to provide additional elasticity, where necessary.
Plastic and metal containers have been replacing glass in many applications where easy handling, low weight and non-breakability are needed. Where metal is used, the internal metal surface of the container must often be coated with a polymer to avoid contact of the packaged content with the metal. Therefore, in the case of plastic packages, and also in case of many metal containers, the contact surface with the packaged content is a polymer. Polymers to date have had varying degrees of inertness to the packaged content which differ from the inertness of glass. In the case of food packages, surface inertness helps diminish potential desorption of packaging material components into the food, to prevent flavor-absorption, to avoid loss of food constituents through the package walls and to avoid ingress of air or other substances from outside the package. All these characteristics of inertness apply to plastic containers; however, some of these characteristics also apply to metal containers which have been internally coated with a plastic or lacquer system.
Refillable plastic packages add a further dimension to inertness requirements because these packages must withstand washing and refilling. Such containers should not absorb contact materials such as the washing agents or foreign materials stored in the container.
Recycling is yet another dimension with mass-produced packages. The reuse of recycled plastic for same purpose, that is to produce new containers ("closed-loop"recycling) is an issue which has attracted much attention, and for PET, this has been achieved to date by depolymerizing the recycled material in order to free it of all trace contaminants, which might otherwise migrate and come in contact with the container content. An impermeable inner layer, which is the purpose of this invention, would enable recycled material to be reused directly for new containers, i.e. without special treatment such as depolymerization, since traces of foreign substances could no longer contact the container's content. This would simplify the "closed-loop"recycling process considerably by obviating the need for depolymerization.
Furthermore, recyclability within established recycling systems, both "open loop", i.e. recycling for other uses, or "closed loop", i.e. reuse for same purpose, is necessary for any mass-produced package. In "open loop" systems, the normal method is to separate, clean and chop up the plastic into small flakes. The flake is then either melted and used for molding other objects, or for fibre production. For this form of recycling, it is important that any contaminant to the main plastic, such as a coating, should effectively be present in negligible quantities and, preferably, be solid and insoluble within the molten plastic so that it can be filtered off prior to sensitive applications, such as fibre-production. PET is also recycled in "closed loop" by depolymerization and it is important that the coating material should be unchanged by this process, be insoluble in the monomers resulting from the process, and be easily separable from these monomers. An inert, thin inorganic coating fulfills these criteria.
Packages utilized for carbonated beverages are also pressurized and must withstand considerable mechanical stress in handling. It is therefore difficult for a single material to provide the necessary mechanical stability and the required inertness. Current plastic packages for carbonated beverages either consist of a single material such as polyethylene terephthalate (PET), or are comprised of multi-layer structures where the middle layers normally provide the barrier properties and the outer layers provide the mechanical strength properties. Such containers are typically produced either by co-injection or co-extrusion.
To date, plastic containers with an impermeable, dense "glass-like" inner surface have not been able to be produced by conventional methods. Prior art is typically comprised of a coating of plastic film using silicon oxides or silicon nitride. However, no method has emerged for satisfactorily coating the inside of bottles or cans, or for providing the degree of surface inertness described.